Schools tend to have a transfer window of a few weeks where you can easily switch courses. My maths dept do an assessment, majorly focused on algebra (we set bridging work so it should all be fresh), and generally it's obvious from that whose algebra isn't up to scratch. We're not allowed to tell kids to leave, but at that point it might be strongly suggested that their talents lie elsewhere. Some stay and then have a miserable time.
Then you get other kids whose algebra is ok and they should be ok, but they just don't do enough work. This is really common. Every year we tell kids that they should be doing an hour of study out of lessons for every hour in lessons and they don't get it. They continue as they did at GCSE, turning up to lessons, half-arsing their homework, spend their study periods hanging out with their mates and then come January exams are shocked when they flunk. For some this is a kick up the arse, for others they say 'oh I just didn't revise enough' then continue as they are to end of Y12 where they then flunk again and then have a bit of a crisis.
The problem with maths A-level is that kids see it as a continuation of GCSE. But at GCSE everything was taught in lessons, they had maybe half an hour of homework a week, and revision time was built into lessons - they probably spent half of Y11 maths lessons revising, they get good maths grades and think that they can do well in maths by turning up to lessons and doing a bit of homework.
Maths A-level however is completely different. There is barely enough time to deliver the content in lessons, there is very little time for practice of methods, barely any time for 'revision lessons' and the vast majority of what used to be done in maths lessons at GCSE now has to be done at home. Kids need to be working through exercises independently, if they find they're not good at something they need to watch videos, do extra work until they are good at it.
We tell them to do it, but it takes a lot for some to realise that they actually need to do it. And then when they do realise, they're behind.
If they hit the ground running with good study habits they'll be in a much better position.